Why is the S&P 500 considered "The Market"?

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S&P 500 has a 99% correlation with the U.S. Stock Market since 1972(As far back as I have S&P 500 data).

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How is the "U.S. Stock Market" defined here and why isn't that used as the benchmark?

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a holdover tradition from the days when compiling that much data was much more difficult and time-consuming than today, i would guess.